[PATCH] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[Marek Polacek <polacek@redhat.com>]

In this problem, we are failing to properly perform copy elision with
a conditional operator, so this:

  constexpr A a = true ? A{} : A{};

fails with:

  error: 'A{((const A*)(&<anonymous>))}' is not a constant expression

The whole initializer is

  TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>

where the outermost TARGET_EXPR is elided, but not the nested ones.
Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
TARGET_EXPRs represent, which is precisely what should *not* happen with
copy elision.

I've tried the approach of tweaking ctx->object, but I ran into gazillion
problems with that.  I thought that I would let cxx_eval_constant_expression
/TARGET_EXPR create a new object only when ctx->object was null, then
adjust setting of ctx->object in places like cxx_bind_parameters_in_call
and cxx_eval_component_reference but that failed completely.  Sometimes
ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
special handling, etc.  I gave up.

But now that we have potential_prvalue_result_of, a simple but less
elegant solution is the following.  I thought about setting a flag on
a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/105550

gcc/cp/ChangeLog:

	* constexpr.cc (struct constexpr_ctx): Add a tree member.
	(init_subob_ctx): Set it.
	(cxx_eval_constant_expression): Don't initialize a temporary object
	if potential_prvalue_result_of says true.
	(cxx_eval_outermost_constant_expr): Adjust the ctx initializer.  Set
	ctx.full_expr.
	* cp-tree.h (potential_prvalue_result_of): Declare.
	* typeck2.cc (potential_prvalue_result_of): No longer static.  Return
	if full_expr is null.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-elision1.C: New test.
---
 gcc/cp/constexpr.cc                           | 33 +++++++++---
 gcc/cp/cp-tree.h                              |  1 +
 gcc/cp/typeck2.cc                             |  4 +-
 .../g++.dg/cpp1y/constexpr-elision1.C         | 53 +++++++++++++++++++
 4 files changed, 82 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 45208478c3f..73880fb089e 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1129,6 +1129,9 @@ struct constexpr_ctx {
   tree ctor;
   /* The object we're building the CONSTRUCTOR for.  */
   tree object;
+  /* The whole initializer expression.  Currently only used when the whole
+     expression is a TARGET_EXPR.  */
+  tree full_expr;
   /* If inside SWITCH_EXPR.  */
   constexpr_switch_state *css_state;
   /* The aggregate initialization context inside which this one is nested.  This
@@ -4700,8 +4703,11 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   new_ctx.ctor = elt;
 
   if (TREE_CODE (value) == TARGET_EXPR)
-    /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
-    value = TARGET_EXPR_INITIAL (value);
+    {
+      new_ctx.full_expr = value;
+      /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
+      value = TARGET_EXPR_INITIAL (value);
+    }
 }
 
 /* We're about to process an initializer for a class or array TYPE.  Make
@@ -6822,6 +6828,14 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    break;
 	  }
 	gcc_checking_assert (!TARGET_EXPR_DIRECT_INIT_P (t));
+	/* This TARGET_EXPR may be nested inside another TARGET_EXPR, but
+	   still serve as the initializer for the same object as the outer
+	   TARGET_EXPR, as in
+	     A a = true ? A{} : A{};
+	   so we can't materialize a temporary.  IOW, don't set ctx->object
+	   to the TARGET_EXPR's slot.  */
+	const bool prvalue = potential_prvalue_result_of (t, ctx->full_expr);
+	gcc_checking_assert (!prvalue || lval == vc_prvalue);
 	/* Avoid evaluating a TARGET_EXPR more than once.  */
 	tree slot = TARGET_EXPR_SLOT (t);
 	if (tree *p = ctx->global->values.get (slot))
@@ -6831,7 +6845,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    r = *p;
 	    break;
 	  }
-	if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
+	if (!prvalue && (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
 	  {
 	    /* We're being expanded without an explicit target, so start
 	       initializing a new object; expansion with an explicit target
@@ -7748,8 +7762,8 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
     }
 
   constexpr_global_ctx global_ctx;
-  constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL, NULL,
-			allow_non_constant, strict,
+  constexpr_ctx ctx = { &global_ctx, nullptr, nullptr, NULL_TREE, NULL_TREE,
+			NULL_TREE, nullptr, nullptr, allow_non_constant, strict,
 			manifestly_const_eval || !allow_non_constant };
 
   /* Turn off -frounding-math for manifestly constant evaluation.  */
@@ -7834,9 +7848,12 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
       if (object && DECL_P (object))
 	global_ctx.values.put (object, ctx.ctor);
       if (TREE_CODE (r) == TARGET_EXPR)
-	/* Avoid creating another CONSTRUCTOR when we expand the
-	   TARGET_EXPR.  */
-	r = TARGET_EXPR_INITIAL (r);
+	{
+	  ctx.full_expr = r;
+	  /* Avoid creating another CONSTRUCTOR when we expand the
+	     TARGET_EXPR.  */
+	  r = TARGET_EXPR_INITIAL (r);
+	}
     }
 
   auto_vec<tree, 16> cleanups;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index d77fd1eb8a9..c3c1f5889a3 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8160,6 +8160,7 @@ extern tree build_functional_cast		(location_t, tree, tree,
 						 tsubst_flags_t);
 extern tree add_exception_specifier		(tree, tree, tsubst_flags_t);
 extern tree merge_exception_specifiers		(tree, tree);
+extern bool potential_prvalue_result_of		(tree, tree);
 
 /* in mangle.cc */
 extern void init_mangle				(void);
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1a96be3d412..6578d5ed6d2 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1383,9 +1383,11 @@ digest_init_flags (tree type, tree init, int flags, tsubst_flags_t complain)
 
    FULL_EXPR is the whole expression, SUBOB is its TARGET_EXPR subobject.  */
 
-static bool
+bool
 potential_prvalue_result_of (tree subob, tree full_expr)
 {
+  if (!full_expr)
+    return false;
   if (subob == full_expr)
     return true;
   else if (TREE_CODE (full_expr) == TARGET_EXPR)
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
new file mode 100644
index 00000000000..b225ae29cde
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
@@ -0,0 +1,53 @@
+// PR c++/105550
+// { dg-do compile { target c++14 } }
+
+struct A {
+  const A *p = this;
+};
+
+struct B {
+  const B *p = this;
+  constexpr operator A() const { return {}; }
+};
+
+constexpr A
+bar (A)
+{
+  return {};
+}
+
+constexpr A baz() { return {}; }
+
+struct E {
+  A a1 = true ? A{} : A{};
+  A a2 = true ? A{} : B{};
+  A a3 = false ? A{} : B{};
+  A a4 = false ? B{} : B{};
+  A a5 = A{};
+  A a6 = B{};
+  A a7 = false ? B{} : (true ? A{} : A{});
+  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+  A a9 = (A{});
+  A a10 = (true, A{});
+  A a11 = bar (A{});
+  A a12 = baz ();
+  A a13 = (A{}, A{});
+};
+
+constexpr E e{};
+
+constexpr A a1 = true ? A{} : A{};
+constexpr A a2 = true ? A{} : B{};
+constexpr A a3 = false ? A{} : B{};
+constexpr A a4 = false ? B{} : B{};
+constexpr A a5 = A{};
+constexpr A a6 = B{};
+constexpr A a7 = false ? B{} : (true ? A{} : A{});
+constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+constexpr A a9 = (A{});
+constexpr A a10 = (true, A{});
+constexpr A a11 = bar (A{});
+//static_assert(a10.p == &a10, ""); // bug, 105619
+constexpr A a12 = baz ();
+//static_assert(a11.p == &a11, ""); // bug, 105619
+constexpr A a13 = (A{}, A{});

base-commit: 97dc78d705a90c1ae83c78a7f2e24942cc3a6257
-- 
2.36.1